This means that QVIS and thus QRAD won't run correctly.
You'll end up with a slow, dark map. Not very pretty.

How did the hole get there? Well that is a very big
question. You may have forgotten or deleted an entire
brush by mistake. You may have moved or subtracted from
a brush by mistake. The list is nearly endless.
In almost all cases it is simple human error. The only
instance I have ever heard a program causing a leak is a map
editor.

Some editors save the numbers that make up your map in their
format (i.e.: .qle, .wc) as FLOATING POINT (with decimals)
numbers. When the file is converted to a .map those
numbers get converted to INTEGER (no decimals) numbers.
This involves rounding the decimal portion up or down, so
the edge of your brush amy have been moved over by 1 unit.
You will not see the change in the editor unless you go to
your maps directory and open the .map file. Ever see
a map that had weird "off 1 unit" corners? Ask Chunk,
he knows this well.

Worldcraft and Qoole are examples of editors that do this.
This is an inconvenience, but there are programming reasons
that the people who made these editors used FLOATING POINT
numbers. To fix the map, open it in the editor like
I (actually Granny)
said. You will see the edges that are off by one unit.
I have gotten into the habit of doing this every so often
because I happen to love Qoole.
It just goes to show that there are trade-offs for every editor.

Well, back to what you are here for. The easiest way
to prevent and find leaks is to compile your map about every
30 minutes using QBSP3 with the -LEAKTEST option. Especially
if you are making new or bigger exterior walls. This
technique merely narrows down the area of a possible leak
to what you were working on last.

If you find a leak, compile the map (don't run QRAD or you
can't see) and go in and take a look at it in the game.
Usually you just forgot a brush or something. If so
you will see a stutter effect where the missing brush is.